Passenger aircrafts are typically equipped with an environmental control system, including an air cycle conditioning system for cooling the aircrew cabins, and other aircraft locations and components. One class of air cycle conditioning systems that are widely used in aircraft to provide cooled air takes advantage of a supply of pressurized air that is bled from an aircraft engine, known as bleed air. Other electrically-driven environmental control systems generally operate by receiving fresh ram air from inlets that are located in at least one favorable position near the ECS equipment bay. The fresh ram air is supplied to at least one electric motor-driven air compressor that raises the air pressure to, for example, the desired air pressure for the cabins. From the at least one air compressor, the air is supplied to an optional ozone converter. Because air compression creates heat, the air is then supplied to an air conditioning pack in which the air is cooled and then transported to the cabin. At least one recirculation system is also provided to recycle air from the cabin and mix it the cooled fresh air.
A conventional auxiliary power unit is a system that provides both air and electricity to the aircraft, duplicates some of the electrically-driven environmental control system in terms of the incorporated components, their functions, and their relative configuration. The auxiliary power unit and the environmental control system are two independent systems, despite the fact that identical components, such as compressors and turbines, are used by both. FIG. 1 is a flow chart depicting an air cycle pathways 25 in an aircraft, including a conventional environmental control system (ECS) 20 a conventional auxiliary power unit (APU) 45, and a redundant ECS 18. As understood from viewing FIG. 1, both ECSs 18 and 20 have substantially identical air cycle pathways.
The ECS 20 will be first described in detail. Details pertaining to the redundant ECS 18 will not be described to the extent that its components are identical to those of the ECS 20. Further, components from the ECS 18 that are substantially identical in their function to those of the ECS 20 are identified with the same reference numerals as those of the ECS 20, with an added prime symbol (i.e. ECS primary heat exchanger 32 and redundant ECS primary heat exchanger 32′).
Air is received by the ECS 20 from both the aircraft exterior as fresh ram air, and from the aircraft fuselage or other interior space as recirculation air. Fresh ram air is supplied from cabin compressors 10a, 10b powered by motors 11a, 11b. The compressed ram air passes through a primary heat exchanger 32 that is disposed in a ram air heat exchanger circuit 56. The ram air heat exchanger circuit 56 has ambient ram air passing therethrough, which cools compressed air in the primary heat exchanger 32, a secondary heat exchanger 34, and an air recirculation heat exchanger 36 that are located in the circuit 56. The ambient ram air is drawn into the heat exchanger circuit 56 through a ram scoop during aircraft flight. When the aircraft is stationary, the ram air heat circuit 56 is driven by an electric fan 54 disposed downstream of the heat exchangers 32, 34, 36 so the heat from the fan 54 is directed overboard rather than into the heat exchangers 32, 34, 36. The ambient ram air in the circuit 56 is cooler than the air passing through the heat exchangers 32, 34, 36, and therefore serves as a heat sink.
After the compressed ram air passes through the primary heat exchanger 32, the air is supplied to a bootstrap air cycle machine, referring specifically to a compressor 40 and turbine 42 that either share the same rotating axis or are otherwise powered and rotated together. The compressor 40 further pressurizes and heats the ram air. The compressed air is then supplied to the secondary heat exchanger 34, causing the compressed air to cool. During normal operation, an altitude valve 60 is closed, causing the air to pass through a re-heater 44 and a condenser 46, and then through a water exchanger 48, which substantially dries the air. From the water exchanger 48, the air is again heated in the re-heater 44, and then the hot and dry air is supplied to the turbine 42. The turbine 42 provides cooled air as a product of air expansion, and forwards the cooled air to the condenser 46, which cools the air further and supplies the air to the cabins in the aircraft fuselage 30. At high altitudes, the altitude valve 60 is opened. The relatively dry and cool air from the high altitude consequently flows from the primary and secondary heat exchangers 34, 32, bypassing the bootstrap air cycle machine, and flows to the cabin. This bypass mode of operation minimizes the supply pressure to the ECS 20 and reduces the required input power to the cabin air compressors 10a-10d. 
A majority of the recirculation air is transferred back to the ECS 20 using a recirculation fan 52, which supplies the recirculation air to the recirculation heat exchanger 36 for cooling. The cooled recirculation air leaves the recirculation heat exchanger 36 and is then mixed with the fresh air being supplied to the aircraft fuselage 30. When limited cooling is required, a recirculation heat exchanger bypass valve 64 is opened, allowing the recirculation air from the recirculation fan 52 to bypass the recirculation heat exchanger 36. Thus, the ECS 20 delivers a dry, subfreezing supply of air to the air distribution system 26 with a significant portion of the ventilation air entering the aircrew cabins being recirculation air.
The combination of the ECS and APU, with their various duplicative components, is expensive and heavy. The architecture as a whole includes a total of eleven turbo machines including the APU power section, two air cycle machines, two ground cooling fans, two recirculation fans, and four cabin pressure compressors. The overall architecture also includes ten heat exchangers, including primary and secondary cooling and reheating and condensing systems for moisture removal and two recirculation coolers. Further, the APU is only used when an aircraft is on ground, and is shut off during a normal flight operation to become a dead weight to the aircraft.
Hence, there is a need to leverage the ECS and APU components in order to reduce the overall weight, volume and energy used during aircraft operation. There is a further need for such systems to include fewer components in order to reduce manufacturing cost.